The correct film formation, curing, adhesion, and absorption of timber coatings are all dependent on and heavily related to the conditions under which the coatings were applied and dried. The long-term performance and the aesthetic qualities of the coating system are therefore linked to the initial application and drying. Improperly cured coatings can be more susceptible to damage from moisture which can be particularly crucial on building sites where joinery items become wet as a result of exposure before installation and certain wet trades that are in progress.
Problems caused by incorrect drying can be as follows:
• Bubbling of the topcoat in wet conditions
• Milky looking areas in translucent and dark opaque finishes
• Excessive water uptake through the coating system in wet conditions
• Poor adhesion and absorption of the coating system
Low temperatures can cause a range of problems such as the following:
• Improper curing – the polymer used to bind the coating film together and hold the pigments in place coalesces to form a continuous protective layer. This process is assisted by the coating film being at a sufficiently high temperature. Also, some constituents in the film cross-link with each other, and this cross-linking process requires a certain temperature to take place. If the cross-linking does not take place, the coating does not build up the necessary protection against moisture.
• Cold joinery items can develop surface dew formation which will hinder absorption of the coating.
• Cold joinery items will lower the temperature of the applied coatings, making them more viscous and less likely to adhere to and absorb into the substrate.
• Cold air temperature and storage conditions will make the coating more viscous resulting in reduced flow and lower absorption.
High temperature can also result in problems:
• Orange peel effect – before the coating film has had sufficient time to flow out to a smooth finish, the skin dries when the coating is still showing the undulations left by the spraying process. This leaves an orange peel effect in the surface.
• Mud cracking – this occurs when there is rapid skin formation over a liquid layer of paint. This can happen in very warm drying areas. The net result is that the liquid or semi-liquid layer is still moving and drying under the skin and cracks appear in the film.
• Dry spray effect – if the drying conditions are very warm and the humidity is too low, one side of the joinery item has started to dry while the other side is still being sprayed. The wet spray layer lands on the dry layer opposite and forms a rough surface that does not flow out.
High humidity during the drying process can result in problems:
• High humidity in the surrounding air prevents moisture from escaping from the coating. This slows down the drying as the moisture has to escape from the wet film. High humidity can be a result of low temperature. The two are linked as warm air holds more water vapour so the relative humidity is lower.
Low humidity during the application process can result in problems as well:
• If the humidity is very low during the application process, spray finishes will suffer from orange peel and potential mud cracking as described previously because of rapid skin formation. A humidity of above 60% at the time of application allows the coatings to flow out better, giving a much more attractive and smooth finish.
• In the case of flow coating or dipping products, higher humidity allows them to run off far more efficiently, giving a better appearance.
Ideal drying conditions are as follows:
Once the drying process is taking place, lowering the humidity and increasing the temperature will reduce drying times. Forced drying of temperatures between 25°C and 35°C and a relative humidity of down to 30% will promote rapid drying. Air movement to remove local humidity is very important, an industrial fan will help with this and is highly recommended.
Assisted force drying can make very rapid drying and curing a possibility. An example of this is catalytic IR (infra-red) drying equipment. When using this equipment, our Hydrolux exterior coating systems can be dried at a rate of 20 minutes per coat.
Accsys Technologies also is the inventor of Tricoya which is supplied through Medite Smart Ply. Accoya extreme is a fibreboard that has the great advantage of no shrinkage and swelling behaviour. However, a correct seal of the end grain is very important as well as a good woodcoating system for the surface. Our Hydrolux woodcoatings have great adhesion on Tricoya and will also minimalize the moisture content in the Tricoya. This will lead to a long outdoor durability. Besides that, our primers will fill the fibreboard and leave a beautiful, smooth surface behind.
Our woodcoatings have been tested by both Accyss Technologies and Medite Smart Ply. Our system recommendations and warranty can be found on their websites.
Modified wood species
There are more modified wood species on the market besides Accoya. The principle of thermal modification (changing the cell structure) of the wood consists of controlled heating to approximately 212˚C. After heating, the wood is carefully (evenly) cooled, and gets changed properties as a result of the heating process or chemical organic treatment with acids, such as increased durability and improved dimensional stability.
Fraké Noir® - Thermally preserved African Limba (Terminalia superba). Due to the thermal treatment of the African hardwood species Limba, the durability class of the wood is increased to durability class I. After the heat treatment, the wood acquires a beautiful colour that is comparable to the colour of walnut, giving it a luxurious appearance. Characteristics of the wood are the black lines and "pinholes" that occasionally occur.
Thermowood® - Thermally modified Finnish spruce (Picea abies). Careful sorting and thermal modification results in a beautiful softwood product with a high durability (II).